A large proportion of packaging adhesives currently in use in the United States and Europe are based on hot melt compositions. Since these adhesives are applied at elevated temperatures, considerable amounts of energy are required to maintain suitable application temperatures. A primary performance parameter is the adhesive's ability to set a bond rapidly and to maintain this bond after a short application of pressure.
As energy costs continue to rise the need increases to replace hot melts with cold or room temperature setting adhesives. By improving, i.e. accelerating, the emulsion setting speed it would be possible to substitute or use water based adhesives in place of the more energy intensive hot melt products.
One method of enhancing adhesive performance parameters such as speed of set, PVC peel strength, water resistance, smoothness (less wood grain raising) and other properties would be to increase the emulsion solids content. In other words, reducing the continuous phase volume or conversely increasing the level of dispersed phase might have an impact. The general concept is to drive the solids up by removing water or continuous phase volume of the polyvinyl alcohol based vinyl acetate-ethylene copolymer emulsions.
Theories on polyvinyl alcohol stabilization rest with steric stabilization via acetyl interactions. Therefore, to obtain optimum stabilization at a given solids one would predict that increasing polyvinyl alcohol molecular weight would enhance steric factors and thus overall latex stability.
Prior act regarding vinyl acetate-ethylene copolymer emulsions prepared in the presence of polyvinyl alcohol includes the following:
U.S. Pat. No. 3,661,696 discloses a process for the production of an aqueous emulsion of an ethylene-vinyl acetate copolymer wherein the polymerization is performed in the presence of a preformed seed emulsion and a minor amount, from 1.5 to 6 wt%, of a protective colloid comprising a mixture of fully and partially hydrolyzed polyvinyl acetate. The resulting emulsion can be used as an adhesive. In Example 9 several emulsion compositions were prepared using a low molecular weight polyvinyl alcohol mixture in combination with a nonionic surfactant. In U.S. Pat. Nos. 3,734,819 and 3,769,151 a similar process is disclosed in which the ethylene-vinyl acetate polymer is prepared in the presence also of a small amount of a vinyl sulfonic acid comonomer or an unsaturated C.sub.3 -C.sub.6 acid, respectively. Similarly, these patent show emulsion compositions prepared using a seed emulsion, the polyvinyl alcohol mixture and a nonionic surfactant.
U.S. Pat. No. 3,692,723 discloses aqueous dispersions and heat melting adhesives comprising same, which dispersions contain a copolymer of ethylene and vinyl acetate wherein the ethylene content is from 30 to 98 wt%, the copolymer having been prepared by a copolymerization process utilizing a particular combination of nonionic emulsifier, anionic enulsifier and protective colloid.
U.S. Pat. No. 3,816,362 discloses a process for preparing a stable aqueous ethylene-vinyl ester copolymer emulsion having an ethylene content of 4 to 20 wt%. Example 1 shows the use of a polyvinyl alcohol, a polyoxyethylene nonylphenyl ether (nonionic surfactant) and sodium dodecyl benzene sulfonate (anionic surfactant).
U.S. Pat. No. 3,827,996 discloses aqueous dispersions of vinyl ester polymers containing as a protective colloid partially hydrolyzed polyvinyl alcohol having an average vinyl acetate content of 5 to 7 mole%. The polyvinyl alcohol may consist of a blend of two or more polyvinyl alcohols, each of which has an average vinyl acetate content different from the average of the blend. Several examples show the use of a partially hydrolyzed and a fully hydrolyzed polyvinyl alcohol in combination with a nonionic surfactant in the preparation of vinyl acetate-ethylene copolymer emulsions.
U.S. Pat. No. 4,043,961 discloses adhesive compositions consisting essentially of an aqueous emulsion of vinyl acetate-ethylene copolymer prepared in the presence of protective colloid comprising fully hydrolyzed vinyl alcohol copolymer containing methyl methacrylate. The examples show the use of such vinyl alcohol copolymer in combination with a medium and/or low viscosity partially hydrolyzed polyvinyl alcohol and a nonionic surfactant.
U.S. Pat. Nos. 4,267,090 and 4,287,329 disclose the preparation of vinyl acetate-ethylene copolymer emulsions in a reaction medium containing a protective colloid and a surface active agent.
U.S. Pat. No. 4,521,561 discloses a vinyl acetate-ehtylene copolymer emulsion exhibiting both partially- and fully-hydrolyzed polyvinyl alcohol compatibility prepared in the presence of a polyvinyl alcohol stabilizing system having an 8-10 mole% residual vinyl acetate content. Since the degree of polymerization of the polyvinyl alcohol affects the viscosity of the emulsion product and is not critical to the invention, polyvinyl alcohols having a degree of polymerization ranging from 200 to 2000 are suitable for use in the invention. In addition to the polyvinyl alcohol stabilizing system, emulsifying agents and protective colloids well known in the polymerization art may also be added in low levels, for example to enhance stability, mentioning polyoxyalkylene condensates.